The present invention relates generally to vehicle tire pressure maintenance, and more specifically, to a tire pressure maintenance device contained on a wheel of a vehicle that automatically regulates and maintains a desired tire inflation pressure or amount of air in a tire.
Under-inflation of vehicle tires is dangerous, deadly and common. Under-inflation is involved in hundreds of thousands of accidents, tens of thousands of injuries and hundreds of fatalities annually in the United States. During 2000, a large number of SUV rollovers and deaths were attributed to significantly under-inflated tires, bringing significant attention to the problem. With the hope of reducing the unacceptably high rate of accidents, injuries, and deaths related to under-inflation, the United States Congress passed the TREAD Act of 2000 that requires a warning system in new motor vehicles to indicate to the operator when a tire is significantly underinflated. Consequently, the National Highway Traffic Safety Administration (NHTSA) proposed a safety standard requiring that, as of 2007, all new passenger cars, trucks, multipurpose passenger vehicles, or busses under 10,000 pounds must be equipped with a tire pressure monitoring system (TPMS) to warn a driver when any tire is under-inflated by 25% or more. The program is estimated to cost well over one billion dollars annually.
However, even if the controversial TPMS program achieves its estimates it will reduce under-inflation related accidents by only about twenty percent. Many industry experts doubt that it will help at all. In contrast, a device that automatically maintains proper tire inflation will eliminate almost all accidents, injuries, and deaths due to under-inflation. In addition, an effective tire pressure maintenance device will improve fuel efficiency by about two percent and will reduce tire tread wear by about ten percent. Such benefits will more than pay for the devices and save billions of dollars annually in the United States if implemented into widespread use.
The temperature of air in a tire has a major effect on the pressure of air in the tire. This factor must be considered in any approach to tire pressure maintenance. FIG. 1 shows how tire pressure varies with temperature according to the ideal gas law. The four pressure-temperature (P-T) lines illustrate the pressure-temperature behavior of a tire filled to 32 psi at air temperatures of 20, 40, 60, and 80° F., assuming a constant tire volume. The four P-T lines represent four different amounts of air in the tire. Ambient temperature variations and tire heating from rolling make tire temperatures and pressures move up and down along the P-T line denoting the amount of air in the tire. A tire will move to a higher P-T line only when air is added and to a lower line only when air is released or leaks out of the tire.
As shown in FIG. 1, the pressure in a tire increases and decreases about 1 psi with temperature increases and decreases of 10° F. Normally, the temperature in the tire increases about 2 to 5 psi above its cold pressure at ambient temperature due to the heat caused by flexing of the side walls and friction from road contact as the car is driven. Most under-inflation is due to inadequate manual tire pressure maintenance. The recommended manual tire inflation procedure is to fill each tire monthly to the manufacturer's recommended cold pressure (MRCP) or placard pressure at ambient temperature. In practice, tires are usually filled less often and also while warm from driving. Further, an ambient temperature drop of 50° F., which is possible within a day and common within a month, reduces tire pressure by about 5 psi. Thus, tire pressures frequently fall 8 psi below the MRCP, typically about twenty-five percent, without considering the normal leak rate of about 1 psi per month.
Two approaches to automatic tire pressure maintenance goals are:                1) Constant Pressure by maintaining the MRCP independent of temperature by adding air when the warm tire pressure is below its warm objective, which is about 3 psi above the MRCP; and        2) Constant Amount of Air by maintaining the amount of air in the tire that produces the MRCP at a selected temperature by adding air any time the tire temperature and pressure fall below the related PT line.        
Both approaches replace air that leaks from tires and assures less variation from the MRCP than manual inflation procedures, with or without a TPMS. Moreover, the constant amount of air approach will minimize deviations from the PT line due to temperature changes and will minimize the amount of air pumped into a tire to maintain the desired inflation pressure.
Many patents have been granted on approaches to automatically maintain the desired inflation pressure in pneumatic tires. None of these approaches address temperature variation significantly. One such approach involves a difficult generation of two continuous out of phase AC voltages that are rectified to provide a continuous DC power source for a DC motor-driven air compressor on the wheel. Another approach discloses a battery-operated compressor contained on a wheel with no practical means for recharging the battery. Another approach requires a TPMS or an on-wheel pressure sensor to send low tire pressure data from the wheel to the vehicle body in order to activate an electromagnet that drives a compressor on the wheel. However, none of these approaches have produced a practical device. Therefore, there exists a need in the art for a tire pressure maintenance means that:                automatically maintains proper tire inflation without operator attention or maintenance;        is small, simple, practical, inexpensive and provides long term reliable operation;        is self-contained on a wheel assembly and operated by wheel rotation;        is fail safe such that failure does not cause deflation or over-inflation of a tire;        alerts drivers to excessive tire leaks or failures; and        provides a higher emergency inflation rate to mitigate leak rates and increase the time for drivers to reach a safe place.        